Proximity-based applications and services represent an emerging social-technological trend. In 3GPP (the 3rd Generation Partner Project), a Proximity Services (ProSe) capability in LTE is introduced in Release-12. It is also aka D2D communications. It would allow the 3GPP industry to serve this developing market, and will, at the same time, serve the urgent needs of several Public Safety communities that are jointly committed to LTE (Long Term Evolution). ProSe normative specification is also important to enable economy of scale advantages, i.e. that the resulting system can be used for both Public Safety and non-Public-Safety services, where possible. In Release-12, two kinds of service are provided, one is D2D discovery and one is D2D communications. The details are given below.
D2D Discovery
FIG. 1 shows scenarios for D2D ProSe where User Equipment, UE1 and UE2 are located in-coverage/out-of-coverage of a cell. When UE1 has a role of transmission, UE1 sends discovery message and UE2 receives it. UE1 and UE2 can change their transmission and reception role. The transmission from UE1 can be received by one or more UEs like UE2.
Table 1 explains the scenarios in FIG. 1.
TABLE 1ScenariosUE1UE21A: Out-of-CoverageOut-of-CoverageOut-of-Coverage1B: Partial-CoverageIn-CoverageOut-of-Coverage1C: In-Coverage-Single-CellIn-CoverageIn-Coverage1D: In-Coverage-Multi-CellIn-CoverageIn-Coverage
In 3GPP, RAN1 and RAN2 will focus on a D2D ProSe discovery mechanism for in-coverage (scenarios 1C and 1D).
For discovery, at least the following two types of discovery procedure are defined for the purpose of terminology definition for use in further discussions/studies.
Type 1: a discovery procedure where resources for discovery signal transmission are allocated on a non UE specific basis.
Type 2: a discovery procedure where resources for discovery signal transmission are allocated on a per UE specific basis:
Type 2A: Resources are allocated for each specific transmission instance of discovery signals;
Type 2B: Resources are semi-persistently allocated for discovery signal transmission.
For Type 1 discovery, the following is agreed in 3GPP: periodic uplink resources are allocated for discovery in a semi-static manner; discovery transmission resource configuration consists of a discovery period, number of sub-frames within a discovery period that can be used for transmission of discovery signals, and for further study number of PRBs (Physical Resource Blocks); for an in-coverage UE, these resources are configured by an eNodeB: allocation can be performed using RRC signaling; resources allocated for discovery within one period of the allocation are TDM and/or FDM into equal sized time-frequency resource blocks that are called “discovery resource”; A discovery resource has a duration of not less than 1 ms and is used for a single transmission of a given discovery MAC (Medium Access Control) PDU (Protocol Data Unit) by a UE; for each discovery period, a UE can transmit on a randomly selected discovery resource.
The eNodeB may provide in SIB (System Information Block): a radio resource pool for discovery transmission and reception in case of Type 1; a radio resource pool for discovery reception of Type 2B.
The basic procedure is: in case of Type 1, a UE autonomously selects radio resources from the indicated Type 1 transmission resource pool for discovery signal transmission; in case of Type 2B, only an “RRC CONNECTED” UE may request resources for transmission of D2D discovery messages from the eNB via RRC, the eNodeB assigns resource via RRC (as baseline, UE releases the transmission resources the latest when the UE enters RRC IDLE or when the eNB withdraws the resource by RRC signaling; in case of Type 2B as baseline radio resource are allocated by RRC, use of activation/deactivation of radio resources using PDCCH is FFS); receiving UEs monitor both Type 1 and Type 2B discovery resources as authorized.
D2D Communications
Scenarios for D2D ProSe direct communications are same as mentioned above in D2D discovery section. When UE1 has a role of transmission, UE1 sends data and UE2 receives it. UE1 and UE2 can change their transmission and reception role. The transmission from UE1 can be received by one or more UEs like UE2.
Public Safety 1: M D2D broadcast communications should be possible irrespective of availability of infrastructure coverage. It is assumed that 1: M D2D broadcast communications should be supported both on a dedicated carrier and on the same carrier as regular LTE. 1:M D2D broadcast communications are one-way and there is no feedback on Layer 2 (MAC/RLC (Radio Link Control)/PDCP (Packet Data Convergence Protocol)). Discovery is not a required step for groupcast and broadcast communications. For groupcast and broadcast, it is not assumed that all receiving UEs in the group are in proximity of each other. Baseline for broadcast communications is that no closed loop physical layer feedback is to be used.
From a transmitting UE's perspective, a UE can operate in two modes for resource allocation: Mode 1: eNodeB or Release-10 relay node schedules the exact resources used by a UE to transmit direct data and direct control information; Mode 2: a UE on its own selects resources from resource pools to transmit direct data and direct control information.
D2D communications capable UEs shall support at least Mode 1 for in-coverage; D2D communications capable UEs shall support Mode 2 for at least edge-of-coverage and/or out-of-coverage.
For D2D broadcast communications, scheduling assignments (SA) that at least indicate the location of the resource(s) for reception of the associated physical channel that carries D2D data are transmitted by the broadcasting UE. The indication of resource(s) for reception may be implicit and/or explicit based on scheduling assignment resource or content.
For Mode 1, the location of the resources for transmission of the scheduling assignment by the broadcasting UE comes from the eNodeB, and the location of the resource(s) for transmission of the D2D data by the broadcasting UE comes from the eNodeB.
For Mode 2, a resource pool for scheduling assignment is pre-configured and/or semi-persistently allocated.
UE on its own selects the resources for scheduling assignment from the resource pool for scheduling assignment to transmit its scheduling assignment.
When transmitting UEs are out-of-coverage, the resources used for D2D broadcast data are selected from a resource pool. The resource pool can be pre-configured, or semi-statically configured. If the resource pool is semi-statically configured, the method of semi-statically configuring the resource pool is for further study.
UEs in-coverage and out-of-coverage need to be aware of a (time/frequency) resource pool for D2D communication reception. All UEs (Mode 1 (“scheduled”) and Mode 2 (“autonomous”)) are provided with a (time and frequency) resource pool in which they attempt to receive scheduling assignments.
In Mode 1, a UE requests transmission resources from an eNodeB. The eNodeB schedules transmission resources for transmission of scheduling assignment(s) and data. The UE sends a scheduling request (dedicated scheduling request, D-SR or Random Access, RA) to the eNodeB followed by a BSR (Buffer Status Report) based on which the eNodeB can determine that the UE intends to perform a D2D transmission as well as the required amount of resources.
In Mode 1, the UE needs to be RRC Connected in order to transmit D2D communications. For Mode 2, UEs are provided with a (time and frequency) resource pool from which they choose resources for transmitting D2D communications. The eNodeB controls whether UE may apply Mode 1 or Mode 2 transmission.
For SA, it was agreed that given a certain SA resource pool and time/frequency resource that is used for a first transmission of an SA message by a UE, the other time/frequency resources used by the same UE for the other transmission(s) of the same SA message within an SA resource period should also be known and fixed in the specification.
The SA resource pool configuration was agreed at RAN1 #78. In general, it was defined in time domain and frequency domain respectively. In time domain, a subframe bitmap is used to indicate the SA resources in time (e.g., 1 indicates subframe has SA resources, 0 indicates subframe has no SA resources). In frequency domain, three parameters are used to indicate the SA resources in frequency: length of SA pool in PRBs, the start PRB index of SA pool and the end PRB index of SA pool. It was also agreed that the each SA occupies 1 PRB and transmits 2 times using RV (Redundancy Version) 0 defined in 3GPP TS 36.212.